The present invention relates to a signal discriminating device having a function to classify input signals supplied thereto based on a spectrum of the signal as information for discrimination, and in particular, to a signal discriminating device suitable for an automatic discrimination of a playback mode of a video tape recorder (VTR) such as one represented by an S-VHS-VTR having two systems of FM standards.
A signal discriminating circuit device of this kind has been described, for example, as a constitution in pages 24 to 27 of a Japanese article "Television Technology", August '87, and such a known technology has been disclosed in the U.S. Pat. No. 4,786,986 assigned to the present assignee (block 12, FIGS. 7-10).
FIG. 7 shows an example the conventional device described in the Japanese article above. The signal discriminating device develops, as represented by the S-VHS-VTR, a signal discriminating function in a VTR having two kinds of FM standards in an FM luminance signal processing such that a difference between reproduced FM signal spectra is detected so as to output a high level or a low level depending on a mode determined by the detection. Description will next be given of a case where the circuit device is applied to the S-VHS-VTR.
In FIG. 7, an input signal is a playback or reproduced FM signal. In the recording and playback processes, the high-frequency output is generally lowered. In consequence, there is provided a peaking circuit 2 to correct the decrease in the high-frequency output, and in general, the peaking circuit 2 is adjusted to develop a peaking characteristic associated with the peaking frequency .function..sub.p .perspectiveto.7 to 8 MHz. A reproduced FM signal corrected by the peaking circuit 2 passes through an amplifier 10 and is then supplied to an S-VHS-band-pass filter (BPF) 11 and a VHS-band-pass filter (BPF) 12. FIG. 3 shows an example of the characteristics of these two band-pass filters 11 and 12.
Let us consider the case where the reproduced FM signal is associated with the S-VHS mode. Since the FM allocation in the S-VHS mode is defined as the top of synchronization=5.4 MHz and the white peak=7.0 MHz, the output amplitude of the S-VHS-BPF 11 becomes greater than that of the VHS-BPF 12. As a result, the output direct-current (DC) potential of a smoothing circuit 13 is higher than that of a smoothing circuit 14.
On the other hand, in a case where the reproduced FM signal is associated to the VHS mode. Since the FM allocation is defined as the top of synchronization=3.4 MHz and the white peak=4.4 MHz, the output DC potential of the smoothing circuit 14 becomes greater than that of the smoothing circuit 13. In this configuration, a comparator 15 effects a compare operation among three input DC potential values. When the output DC potential of the smoothing circuit 13 takes the highest value, the output from a hold controller 17 indicates the S-VHS mode (let us assume here the output is at a high level in this case). When the output DC potential of the smoothing circuit 14 takes the highest value, the output from a hold controller 17 is set to a lower level and hence indicates the VHS mode. When a reference potential 16 separately set takes the highest value, the previous value of the output from the hold controller 17 is kept unchanged. FIG. 8 shows a hold control characteristic of the output mode of the hold controller 17 when the amplitude and frequency of the input signal to the amplifier 10 are changed. In FIG. 8, in a range of the amplitude of the input signal not exceeding 100 mVpp, the value of the output mode is kept unchanged. That is, the range is a dead zone for the input signal. The width of the amplitude dead zone can be arbitrarily set by changing the value of the reference potential 16. When the reproduced FM signal has a small amplitude, a wrong discrimination is likely to occur due to noise and an external disturbance; however, the erroneous discrimination can be prevented by appropriately setting the amplitude dead zone as described above.
In the technology above, in each circuit constituting the amplifier 10, the S-VHS-BPF 11, and the VHS-BPF 12, there exists a dynamic range; in consequence, also for the overall signal discriminating device, there exists a dynamic range for the input signal amplitude in which an appropriate discriminating operation can be achieved. On the other hand, due to causes such as a deviation in the tape head system and a dispersion or unevenness in the gain of the playback preamplifier, the amplitude dispersion of the reproduced FM signal becomes quite large, and hence there exists a fear that the dynamic range of the signal discriminating device is easily exceeded.
In the hold control circuit 17, the previous value of the discrimination output is generally held in a form of an electric charge by use of a capacitor. In consequence, the leakage of the charge cannot be avoided, and in order to prevent the effect thereof, it is necessary to increase the size and capacity of the capacitor. In this case, however, the value of the time constant increases and hence the response speed of the circuit is lowered, which leads to a problem in the practical operation. The discriminating device according to the U.S. Patent above also employs a capacitor to hold the signal and hence is attended with the similar problem.